GridVoxel, Ray: changed opacities from ray-based to grid-based quantities

Making them ray-based allows us to deal with anisotropic sources and
sinks and is therefore more general. But I have no plans to implement,
e.g., partial redistribution any time soon so it's a pain to deal with
right now.
Merge branch 'change_opacities_to_grid_values' into develop
wavelength_grid: changed all wavelength grids from maps to vectors

We can just use the index as a "hash". I don't know why I went to so
much trouble to abstract away the indices. Those map lookups are
expensive anyway.
GridWavelengthPoint: removed no-longer-true comment about thermalization parameter
Grid, GridWavelengthPoint: added vector of pointers to AtomicLines nearby a certain wavelength

This should make the opacity calculation go much faster, so we don't
have to sum over every single line at every single wavelength point.
However currently this vector of pointers is unused and the routine
"find_nearby_lines()" is not yet called.